1. Field of the Invention
The present invention relates to a demodulation method for demodulation by converting analog signal obtained by reading information memorized in a recording medium to digital signal so as to generate data representing the information and a demodulator.
2. Description of the Related Art
Since before, there has been an information recording/reproducing apparatus for recording information in a recording medium and reproducing information by reading it from that recording medium. The information recording/reproducing apparatus includes, for example, a magnetic disc unit in which information is recorded on a magnetic disc and the information is reproduced by reading out from the magnetic disc.
FIG. 9 is an outline diagram showing a magnetic disc unit, which is an example of such an information recording/reproducing apparatus.
A magnetic disc 10 is rotated in a direction indicated by an arrow A around a center shaft 11 by a spindle motor (not shown).
An actuator 20 is rotated around a rotation shaft 21 so as to move a magnetic head 30 provided on a front end thereof in a radius direction (a direction of arrow B) of the magnetic disc 10 along the surface of the magnetic disc 10. The magnetic head 30 records information based on signal sent from a signal recording/reproducing portion 40 into the magnetic disc 10 and picks up information recorded in the magnetic disc 10 and sends it to the signal recording/reproducing portion 40.
The signal recording/reproducing portion 40 receives data signal carrying recording information form outside when the information is recorded to the magnetic disc 10 and carries out a predetermined processing including run length limited (RLL) coding. The magnetic head 30 is driven according to a signal after the processing, so as to record information in the magnetic disc 10. On the other hand, when the information is reproduced from the magnetic disc 10, error correction processing and RLL decoding processing are carried out on signal picked up by the magnetic head 30 and sent out of this magnetic disc unit.
Servo information for controlling the position of the magnetic head 30 with respect to the magnetic disc 10 as well as ordinary information for read/write are recorded in the magnetic disc 10. This servo information is picked up by the magnetic head 30 and sent to the position control portion 50 through the signal recording/reproducing portion 40. The position control portion 50 controls an operation of the actuator 20 for the magnetic head 30 provided at a front end of the actuator 20 to move with respect to the magnetic disc 10 to a desired position based on the information.
FIG. 10 is a block diagram of a conventional demodulator which composes a signal reproducing portion for reproducing data indicating information recorded in the magnetic disc 30 from signal picked up by the magnetic head 30, in the signal recording/reproducing portion 40 of the magnetic disc unit shown in FIG. 9.
Analog signal obtained by a signal pickup of the magnetic head 30 is inputted to gain control amplifier (GCA) 101 capable of changing amplification factor in the demodulator 100 and amplified appropriately. Output analog signal from the GCA 101 is inputted to an analog equalizer 102 and equalized by this equalizer. After that, this signal is converted to digital signal by the A/D converter 103 and maximum likelihood is detected by a maximum likelihood detector 102. A result of maximum likelihood detection is RLL decoded by a RLL decoder. Then, an error is corrected by error correction code (ECC)106 so as to reproduce right data.
Here, the digital signal outputted from the A/D converter 103 is also inputted to an automatic gain control (AGC) 107 and phase locked loop (PLL) 108.
Prior to description of an operation of the AGC 107 and PLL 108, first, data structure of information to be picked up from the magnetic disc 30 will be described.
FIG. 11 is a diagram showing data structure of information to be picked up from the magnetic disc.
First, acquisition portion GAP is disposed and next, sync byte portion SB for indicating a start of proper data is disposed followed by the proper data.
The AGC 107 and PLL 108 shown in FIG. 10 use signal from the acquisition portion GAP. The AGC 107 adjusts amplification factor of the GCA 101 based on the output digital signal from the A/D converter 103 of the acquisition portion GAP so that appropriately amplified signal is outputted from the GCA 101. The PLL 108 generates a clock signal which is a reproduction of a clock upon recording of information, based on the output digital signal from the A/D converter 103. Signal amplified appropriately depending on the size of signal picked up by the magnetic head is outputted from the GCA 101 to the sync byte portion SB at a timing in which actual data is inputted. A/D conversion is carried out by the A/D converter 103 at a clock reproduced to be same as the clock upon recording information.
Because in recent years, high density recording has been accelerated in information recording/reproducing apparatus such as magnetic disc unit, noise increases in the acquisition portion GAP resulting therefrom, so that a minute defect in a recording medium affects relatively largely. If the defects in the recording medium are accumulated in the recording portion of the acquisition portion GAP, adjustment of the amplification factor and reproduction of the clock by the AGC 107 and PLL 108 are not carried out excellently. Consequently, so-called cycle skip and A/D converter clamp occur so that a long burst error may occur. If the burst error occurs, correction is disabled even if a high performance ECC 106 is employed, so that accurate data reproduction is disabled. Thus, the performance of the demodulator is determined depending on how accurately the AGC and PLL are operated.
In views of the above-described problem, the present invention intends to provide a demodulation method and a demodulator capable of obtaining correct data even if S/N ratio is lower than conventional.
To achieve the above object, according to an aspect of the present invention, there is provided a demodulation method for demodulation by converting analog signal carrying a first clock of a predetermined first frequency obtained by reading information recorded in a recording medium to digital signal so as to generate data representing the information, wherein
the analog signal is converted to a first digital signal by over-sampling synchronous with a second clock of a second frequency higher than the frequency of the first clock and
a phase error of the first clock with respect to the second clock is obtained based on the first digital signal.
According to another aspect of the present invention, there is provided a demodulator for demodulation by converting analog signal carrying a first clock of a predetermined first frequency obtained by reading information recorded in a recording medium to digital signal so as to generate data representing the information, the demodulator comprising:
an A/D converter for converting the analog signal to a first digital signal by over-sampling synchronous with a second clock of a second frequency higher than the frequency of the first clock;
a buffer for storing the first digital signal; and
an operating portion for obtaining a phase error of the first clock with respect to the second clock based on the first digital signal stored in the buffer.
According to the demodulation method and demodulator of the present invention, over-sampling is carried out synchronously with a clock (second clock) having a higher frequency (second frequency). A first digital signal obtained by the over-sampling is converted to a second digital signal synchronous with a clock (first clock) of a proper frequency (first frequency). Then, the second digital signal obtained in that way is decoded. Therefore, the necessity of the acquisition portion is eliminated thereby formatting efficiency being improved.
According to the present invention, the clock frequency (first frequency) of the first clock and phase are extracted by computation on data. Therefore, even if the S/N ratio is low, it is possible to eliminate a burst error which is generated conventionally when leading into the PLL (arrival to proper operation) is incomplete, so as to achieve normal operation of the demodulator.
Meanwhile, the AGC and GCA shown in FIG. 10 can be adjusted by adjusting data value corresponding to an amplification factor of the GCA on data because the present invention depends on mainly computation on data. Therefore, for example, the amplification factor of the GCA can be maintained at a fixed value while omitting the AGC, instead of changing the amplification factor of the GCA largely.
Preferably, in the demodulator of the present invention, the operating portion comprises:
a Fourier transforming portion for Fourier-transforming the first digital signal;
a clock extracting portion for obtaining the first frequency and an initial phase of the first clock with respect to the second clock from Fourier transformation signal obtained from the Fourier transformation by the Fourier transforming portion; and
a phase error computing portion for obtaining a phase error of each clock pulse of the first clock with respect to the second clock based on the first frequency and the initial phase obtained by the clock extracting portion.
In this case, the clock extracting portion may obtain the first frequency by linear estimation of amplitude values of frequencies before and after the first frequency based on amplitude information of the amplitude information and phase information composing the Fourier transformation signal, and may obtain the initial phase by linear interpolation using the phases of frequencies before and after the first frequency based on the phase information.
Further, the operating portion may further comprise an interpolation computing portion for obtaining a second digital signal synchronous with the first clock by interpolating the first digital signal based on phase error information obtained by the phase error computing portion.
For example, by this computation, the first clock can be obtained at a sufficient accuracy, and the second digital signal, which is a proper signal, can be generated from the first digital signal obtained by over-sampling at a sufficient accuracy.
Further, preferably, the demodulator of the present invention is provided with an equalizer for equalizing analog signal obtained by reading information stored in the recording medium at a pre-stage of the A/D converter. Further, it is permissible to provide a low-pass filter portion for carrying out low-pass filtering on the analog signal obtained by reading information stored in a recording medium, at a pre-stage of the A/D converter. Further, it is also permissible to provide a FIR filter conforming to the second clock between the A/D converter and buffer so as to progress the equalization. Further, it is permissible to provide a FIR filter conforming to the first clock between the operating portion and demodulating portion so as to progress the equalization.
In any case, demodulation to correct data is urged.
Further, in the demodulator of the present invention, the above-described demodulating portion may be provided with an error correction code portion which acts as a buffer at the same time.
Sharing the buffer leads to reduction of the circuit size.
As described above, according to the present invention, it is possible to raise a probability that correct data can be obtained from signal having low S/N ratio, as compared to the conventional technology.